Hydroxyapatite (Ca10(PO4)6(OH)2), which is obtained by sintering a calcium phosphate composition, has a composition close to inorganic components of bones and teeth and has a bioactivity, which is a property of bonding directly to bones. Therefore, its use as a material for repairing bone defects or bone voids has been reported. When a bone defector a bone void is filled up with a material made of such hydroxyapatite obtained by sintering, it can be connected directly to the bone, but it has been reported that bone formation due to remodeling of the bone fails to occur. Therefore, it is reported that hydroxyapatite obtained by sintering a calcium phosphate composition only plays a role of a space-making material which only exists as it is and that it is difficult to have a bone forming ability even if it stays inside the body for a long period of time.
On the other hand, it is known that among calcium phosphate compositions, a cement type of composition, that is, a calcium phosphate composition having setting property is converted gradually to living body-absorbable hydroxyapatite in a living body or in an oral cavity and moreover it can integrate with a biological hard tissue while maintaining its form. Such a calcium phosphate composition is not only superior in biocompatibility but also has formability, and it is replaced with bone itself by the bone formation originating at absorption of osteoclasts. Therefore, its application as a medical and dental material is expected.
For example, Japanese Patent No. 3017536 (patent document 1) discloses that a mixture of tetracalcium phosphate and anhydrous calcium hydrogen phosphate reacts in the presence of water to form hydroxyapatite. It is reported that when the hydroxyapatite comes into contact with a biological hard tissue, it can replace bone gradually. However, the resulting hydroxyapatite does not necessarily have good mechanical strength, and it was sometimes difficult to be applied to a site where a load is to be added. In addition, the rate of its replacement with bone is also insufficient. Therefore, further improvement has been desired.
Non-patent document 1 discloses that a calcium phosphate cement made of tetracalcium phosphate and anhydrous calcium hydrogen phosphate exhibits good filling ability and it has high compatibility with the periapical tissues when used as a root canal filler. Cases where zirconia was used as a radiopaque material are also disclosed. However, a hardened calcium phosphate cement obtained by setting such a calcium phosphate cement does not necessarily have satisfactory mechanical strength, and therefore an improvement has been desired.    Patent document 1: Japanese Patent No. 3017536    Non-patent document 1: Akiyoshi Sugawara et al.,    “Histopathological Reaction of a Calcium Phosphate Cement Root Canal Filler”, Journal of Hard Tissue Biology, 4 (1): p 1-7, 1995